Coronary heart disease (CHD) continues to be the leading cause of death in the United States. Children with elevated cholesterol levels are especially vulnerable and are at high risk for early onset CHD. Endothelial dysfunction, the earliest phase of coronary atherosclerosis, is present in children with elevated cholesterol levels as early as 8 years of age. The long term objective of this study is to develop dietary interventions for the prevention and treatment of endothelial dysfunction in children with common lipid disorders including familial hypercholesterolemia (FH) and familial combined hyperlipidemia (FCH). Increasing evidence suggests that the Mediterranean diet and ?-3 fatty acids found in fish have cardioprotective effects. The specific aims of the study are to: determine whether a Mediterranean diet alone or combined with ?-3 fatty acids (eicosapentaenoic acid, docosahexaenoic acid or eicosapentaenoic & docosahexaenoic acids) improve endothelial function; evaluate the effects of the dietary interventions on lipids and lipoprotein subclasses; evaluate the effects of the dietary interventions on biomarkers for oxidative stress and inflammation. This study is a randomized, double-blind, placebo-controlled clinical trial that includes 102 children (ages 8-18) treated with the Mediterranean diet and ?-3 fatty acid supplements. A dietary educational behavioral intervention will be conducted over 6 months with individual counseling, group sessions and an interactive web-based program. Endothelial function will be measured non-invasively by high resolution ultrasound of the brachial artery at baseline, 6, 12, 18 and 24 weeks. This study is unique because it is a new intervention designed for children at high risk for early CHD with a dietary component and supplementation with ?-3 fatty acids. If effective, this intervention would be broadly applicable and lend valuable insight about dietary therapy to prevent the progression of CHD in hyperlipidemic children.